1. Field of the Invention
This invention relates to methods of producing yokes each constituting a field section of a rotary electric machine, and more particularly it is concerned with a method of producing a yoke suitable for use in a DC machine for an automotive vehicle, such as a starter.
2. Description of the Prior Art
A yoke constituting a field section of a rotary electric machine is cylindrical in shape and securely fixed between two frames forming an outer shell of the machine, such as a DC machine, when assembled. A yoke usually has two abutting surfaces on axial opposite ends thereof which are to be positioned against the frames. In producing a yoke, it has hitherto been mandatory to finish the yoke in such a manner that its inner diameter has high dimensional accuracy and its abutting surfaces are parallel to each other and spaced apart a predetermined distance.
In one method of producing this type of yoke known in the art, a strip of suitable material is rounded into a cylindrical shape which is welded at opposite sides, the cylinder is subjected to ironing at its outer or inner periphery so that its inner diameter will have an accurate value, and then abutting surfaces to be positioned against the frames are machined. The method described above has generally been used for producing yokes of high precision finishes. This method has, however, the disadvantage that it has many process steps resulting in increased costs. Also, machining of the abutting surfaces has produced waste of material.